1. Field of the Invention
The present invention relates to a process for producing a thin-plate form glass molded body, wherein a melted glass is pressed, and a process for producing a disc form magnetic recording medium.
2. Description of the Related Art
A typical example of information recording media having a recording layer that makes use of a nature of magnetism, light, photomagnetism or the like is a disc form magnetic recording medium. As the substrate of the disc form magnetic recording medium, an aluminum substrate has widely been used hitherto. In recent years, however, following a request for a decease in the floating amount of a magnetic head for improving the recording density of the medium, attention has been paid to a glass substrate that can be expected to cause a decrease in the magnetic head floating amount since the substrate is better in flatness and smoothness and the number of surface defects is also smaller than aluminum substrates.
Such a glass substrate is produced by subjecting a plate form glass molded body, which is called a blank material, to polishing process and others. The plate form glass molded body is produced by press molding, a method of cutting a glass plate produced in a floating manner, or some other method. Attention is paid, in particular, to a technique of pressing a melted glass directly (which may be referred to as “direct press” hereinafter) since a particularly high productivity can be expected.
However, when a plate form glass molded body is produced by direct press, the glass is rapidly cooled to be easily cracked or turned into an undesired state since a melted glass, which has a high temperature, is used as a raw material. Thus, Japanese Patent Application Laid-Open No. 2008-174402 discloses a technique of adjusting the temperature (° C.) of a pressing member into the range of “Tg−20” to “Tg+100” wherein Tg represents the glass transition point of a glass to be used, thereby ensuring the quality of a plate form glass molded body.
Incidentally, in the production of a substrate for a disc form magnetic recording medium, the thickness of a blank material for direct press is approximated to the thickness of the substrate as much as possible, thereby improving the ratio of the product amount to the use amount of a raw material to decrease producing costs. In recent years, however, requests for a decrease in costs have been rising up. Thus, it has been desired that a blank material should made even thinner and the production amount per unit period should be made higher (higher-speed press should be realized). However, in the case of using direct press, the following situation is easily caused when the speed of the press is made higher: as the thickness of a target plate form glass molded body is made smaller, the molded body is more easily cracked, deteriorated in flatness, or turned in an undesired state.